1. Field of the Invention
This invention relates to thermally stabilized adhesive compositions of thermoplastic segmented copolyester elastomers and at least one compatible low molecular weight thermoplastic resin, and to methods for preparing such compositions.
2. Description of the Prior Art
Copolyesters, and particularly segmented copolyester elastomers, are used in the formulation of adhesives such as those useful as hot melt adhesives. Such adhesive compositions are described in U.S. Pat. No. 3,832,314. The compositions of this patent have good bond strength as hot melt adhesives. In order to provide good adhesive properties the viscosity of the adhesive blend must be maintained at a relatively constant level. It has been found that at elevated temperatures, particularly in the range of 170 to 200.degree.C., over a period of several hours the aforementioned adhesive compositions lose viscosity and hence their bonding properties are reduced. Stabilizers or antioxidants such as tetrakis[methylene-3-(3',5'-ditertiary-butyl-4'-hydroxyphenyl) propionate] methane, and phosphite ester compounds, have been incorporated in compositions containing segmented copolyesters, but the known stabilizers have not proved to be very effective over extended periods of time, e.g., 2 hours and more.
Other stabilizers have been added to adhesive compositions prepared from thermoplastic segmented copolyester elastomers and thermoplastic resins. A particularly effective stabilizer is an alkaline earth oxide, 0.1 to 4.0 percent by weight, based on the weight of elastomer and resin, as disclosed in Eastman, U.S. Ser. No. 470,266, filed Feb. 20, 1974. Improved adhesive compositions according to Eastman also can contain 0.1 to 1.0 percent by weight, based on the weight of elastomer and resin, of a substantially linear polycarbodiimide having an average of at least two carbodiimide groups per molecule.
While the Eastman stabilizers have proved beneficial, it has been found that the shelf-life of the adhesive composition is short resulting in a loss of stability, particularly if the adhesive composition is in pelletized form.